The use of narrowband Fabry Perot etalons for spectral analysis is known in the art, as evidenced by those described by R. Russel Austin in "Solid Fabry-Perot Etalons as Narrow Band Filters" (Electro Optical System Design, 6, 32, July 1973, pp. 32-37), Adrian E. Roche and Alan M. Title in "Ultra Narrow Band Infrared Filter Radiometry", Second Joint Conference on Sensing Atmospheric Pollutants, -ISA-JSP 6656, Washington D.C., Dec. 10-12, 1973, pp. 21-24. Narrowband etalons are used in such applications as Fraunhofer Line Discriminators, as described in "The Fraunhofer Line Discriminator MK II" by James A. Plascyk and Fred C. Gabriel (IEEE Transactions on Instrumentation and Measurement, Vol. IM-24, No. 4, December, 1975, pp. 306-313), and in the Hydrogen Alpha Telescope launched by NASA.
Most prior art Fabry Perot etalons filter out only a single, narrowband line. However, since the etalon exhibits a periodic channel spectrum the periodicity of channel spectra can be matched to nearly periodic spectra over a narrow spectral region. When the source spectra is notably aperiodic, the etalon can be matched to only two lines. Furthermore, if the source lines are widely separated, degradations in the etalon finesse typically allow the etalon to be used for only one line. One common example concerns the Fraunhofer lines in the atmosphere. These lines are not only aperiodic, but are also widely spaced apart. Therefore, three separate etalons were required to be used in the Fraunhofer Line Discriminator referred to above.
As employed herein, the term "etalon" is intended to encompass an optical device or element having two partially reflecting surfaces that are parallel to each other to optical tolerances. The space between the two reflecting coatings can be air or an optical material, and can be thick or thin. The thicker the spacer, the higher the resolution of the etalon. FIG. 1a shows a "solid" etalon where the spacer is a thick optical material labeled substrate. When the spacer is solid and thin, the etalon assumes the form of an interference filter.
FIG. 1a illustrates a flat multi-line etalon 1 comprised of a spacer material, or substrate 2, and coatings 3 and 4. The transmission characteristics of the etalon I are designed to be nominally matched to atmospheric or laser spectral lines. FIG. 1b illustrates the periodic spectral lines passed by the etalon 1 (transmission peaks) and also illustrates typical aperiodic atmospheric spectral lines. The prior art etalon 1 does not exhibit dispersion (.phi.=0). That is, the prior art etalon does not generate phase shifts as a function of wavelength. As used here, dispersion in an etalon coating is defined as a variation of phase shift upon reflection with wavelength, frequency or period. As a result, the periodic etalon "walks off" of the aperiodic atmospheric spectral lines, which are affected by molecular dispersion. This results in a failure of the etalon 1 to pass the atmospheric lines of interest and a resulting failure to detect the presence of these lines.
Alternately, one can broaden the width of the filter lines to pass the molecular lines, but this degrades the effectiveness of the filter.
In greater detail, a high finesse etalon produces multiple transmission peaks whose locations are given by: EQU .phi.+.pi.=2.PSI.l; where
.PSI.=2kd=4 nd.pi.cos.theta.'/.lambda.=phase of etalon spacer;
l=integer;
n, d, .theta.'=etalon index of refraction, thickness, and internal angle, respectively;
.lambda.=wavelength of Nth peak; and
.phi.=the phase of the etalon coating.
The etalon "finesse" is a measure of etalon quality and may be expressed as a ratio of line spacing to line width. In other words, the etalon finesse is a function of etalon reflectivity so that as reflectivity increases, so does the finesse.
Once the etalon 1 spacer material 2 is chosen, the index of refraction and internal angle are determined. The wavelengths of the desired transmission peaks are assumed to be given a priority. The etalon 1 thickness is chosen so as to set the free spectral range and to locate one line, or transmission peak. However, if the lines are not periodic the etalon, having a non-dispersive coating, can be matched to only two lines.
It is thus one object of the invention to provide a dispersive coating to specify the transmission peak characteristics of a multiple transmission peak (multi-peak) etalon.
It is another object of the invention to provide improved coatings for multi-peak etalons, the coating providing a controlled and prescribed dispersion characteristic for an etalon, even when the peaks are far apart.
Another object of the invention is to provide a dispersive rugate coating fabricated so as to define the transmission peak characteristics of a multi-peak etalon.